Some people know a lot of words but they can never remember them when they need to use them. These people have a very wide passive vocabulary. That means that they know a lot of words, but cannot find them quickly.
The reason that I think this happens is to do with the different ways our brains are programmed. Some people have very wide vocabularies – they know a lot of words and they are sharp enough to remember them quickly. These people would be great contestants on quiz shows.
Not everyone thinks in the same way. In the quiz show analogy, some people watch the quiz and know the answer, but they can never remember the answer quickly enough. These people are much slower thinkers. The information is in their brain somewhere, however it takes much longer to find it.